Petrology, geochronology, and geophysical characterization of
Mesoproterozoic rocks in central Illinois, USA
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Abstract
The Precambrian basement rocks of the Eastern Granite-Rhyolite Province (EGRP) in central Illinois (midcontinent
region of North America) exhibit a complex history of early volcanism, granite emplacement, and
intrusion of mafic rocks. A comprehensive suite of dedicated petrographic analyses, geophysical logs, and drill
core from four basement-penetrating wells, two-dimensional and three-dimensional seismic reflection data, and
U-Pb age data from the Illinois Basin–Decatur Project (IBDP) and Illinois Carbon Capture Storage (ICCS) Project
site provide new constraints for interpreting the Precambrian basement of the Illinois Basin. These new data
reveal the basement to be compositionally and structurally complex, having typical EGRP felsic volcanic rocks
intruded by the first reported gabbro in the Precambrian basement in Illinois. Zircons (n ¼ 29) from rhyolite give
a U-Pb weighted mean average age of 1467 9 Ma. Zircons (n ¼ 3) from a gabbro dike that intrudes the rhyolite
yield a concordia age of 1073 12 Ma, which corresponds to Grenville-age extension and represents the first
Grenville-age rock in Illinois and in the EGRP. A high-resolution three-dimensional seismic reflection volume,
coincident with the four wells, provides a context for interpreting the petrological data and implies a high degree
of heterogeneity for basement rocks at the IBDP–ICCS site, as also shown by the drill cores. The occurrence of
Grenville-age gabbro is related to a prominent bowl-like structure observed on local two-dimensional seismic
reflection profiles and the three-dimensional volume that is interpreted as a deep-seated mafic sill complex.
Furthermore, heterogeneities such as the brecciated EGRP rhyolite and later gabbro intrusion observed in the
basement lithology at the IBDP–ICCS may reflect previously unknown distal elements of the 1.1 Ga Midcontinent
Rift in the EGRP and more likely Grenville-age extension.
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